Currently, the composite membrane is extensively applied in water treatments, e.g. such as ultrafiltration, microfiltration, reverse osmosis, forward osmosis, etc.
In a traditional composite membrane structure, the upper layer is a porous layer, and the lower layer is a reinforced layer in order to increase mechanical strength. This reinforced layer can be a woven fabric or a non-woven fabric, even other materials capable of reinforcement. However, when the reinforcement layer and the polymer solution of the upper layer thereon are combined, due to issues such as viscosity, the polymer solution for of the porous layer penetrate into the reinforced layer, such that the reinforced layer with a large-resistance interface layer is formed after film-forming. The interface layer causes an increase in the resistance of the composite membrane such that the membrane performance is affected.
This phenomenon is more severe in forward osmosis techniques. Since liquid flows at both sides of the composite membrane for forward osmosis, very severe internal concentration polarization occurs at the side of the reinforced layer, and the interface layer having a large resistance worsens the internal concentration polarization issue.
Increasing the viscosity of the polymer solution of the porous layer can prevent the porous layer from penetrating into the reinforced layer and forming an interface layer having a large resistance. However, changing the viscosity of the polymer solution may result in a denser membrane structure having greater resistance, and water flux is reduced as a result. Therefore, an interface layer having a high resistance while maintaining water flux is an important topic in forward osmosis membranes.